Acicular alloy containing magnetic recording medium

ABSTRACT

An acicular alloy base magnetic powder comprising iron and cobalt, which has an average particle long axis size of not larger than 0.25 μm, an axial ratio of from 4 to 8, a cobalt content of from 8 to 50% by weight based on the weight of iron and saturation magnetization of at least 120 emu/g after being kept standing at the temperature of 60° C. and relative humidity of 90% for a week, which has improved corrosion resistance and can provide a magnetic recording medium suitable for recording in the short wavelength range.

This application is a continuation of application Ser. No. 07/429,642filed on Oct. 31, 1989, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to acicular alloy magnetic powder, amethod for producing the magnetic powder and a magnetic recording mediumcomprising the magnetic powder. More particularly, it relates toacicular fine particle magnetic powder made of an alloy comprising ironand cobalt, a method for producing such fine particle magnetic powderand a magnetic recording medium comprising such fine particle magneticpowder and having improved corrosion resistance.

2. Description of the Related Art

Since metal iron base magnetic powder has larger coercive force andsaturation magnetization than iron oxide base magnetic powder and issuitable for high density recording, it is now practical to be used forproducing a magnetic recording medium.

Since the particle surfaces of metal iron base magnetic powder are veryactive and easily corroded, its handling is difficult, and further themagnetic recording medium comprising the metal iron base magnetic powdersuffers from decrease of output characteristics under high temperatureand high humidity conditions. This is apparent from the fact that thesaturation magnetization of the metal iron base magnetic powder greatlydecreases in several hours when the magnetic powder is placed in anatmosphere of 60° C. and humidity of 90%.

To improve the corrosion resistance of the metal iron magnetic powder,it is proposed to use an alloy of iron with other metal such as cobaltwhereby a passive state film is formed on the particle surface.

Standard methods for producing the alloy base magnetic powder include:

(1) reduction of a co-precipitated material prepared from an iron saltand a cobalt salt which are added to an aqueous solution of oxalic acid:

(2) thermal reduction of iron oxide particles on which surface cobalt isdeposited:

(3) addition of a reducing agent to a solution comprising an iron saltand a cobalt salt:

(4) evaporation of metal in an inert gas to cause collision of theevaporated metal with the gas molecules: and

(5) reducing iron chloride and cobalt chloride both in vapor states in amixture of hydrogen with nitrogen or argon to form metals.

In the method (1), control of the composition of the particles isdifficult. In the method (2), since the cobalt compound is formed on theiron particles, it is difficult to maintain the acicular form. In themethods (3), (4) and (5), the produced magnetic powder is not acicularbut in the form of chained beads and does not have satisfactoryorientation.

To overcome the above problems, it is proposed to thermally reduceacicular goethite particles which contain cobalt and are prepared fromalkaline aqueous suspensions of the iron salt and the cobalt salt.

Although the alloy base magnetic powder produced by the above method hasbetter corrosion resistance than the conventional metal iron basemagnetic powder, the content of cobalt in the powder does not exceedabout 7% by weight and at such low cobalt content, satisfactorycorrosion resistance cannot be achieved. The reason for this has notbeen made clear, but may be attributed to insufficient formation of thepassive state on the particle surfaces because of shortage of cobalt inthe metal magnetic powder.

Then, the present inventors thought that it would be necessary toprotect the passive state film or to supply a sufficient amount ofcobalt, and performed the following experiments.

To supply the sufficient amount of cobalt in the above method comprisingthermally reducing the acicular goethite which contains cobalt, anexcess amount of the cobalt salt was added to the aqueous suspension inorder to increase the cobalt content in the produced magnetic powder.However, the particle shape or uniformity of the composition weredisturbed, that is, the shape of goethite particle was deformed, orparticles with irregular shapes were contained in the goethite powder.Therefore, the sufficiently large amount of cobalt cannot be introducedin the metal magnetic powder.

The present inventors investigated causes for such phenomena, and it isfound that the problems will not be solved by the conventional methodsin which the cobalt salt is added to the suspension during formation ofthe goethite powder. The reasons for this are as follows:

First, since the iron constituting the goethite is trivalent and is notequal to the valency of cobalt which is divalent, the iron ions and thecobalt ions cannot be freely exchanged. Second, the cobalt concentrationin the aqueous suspension may control a growth rate of the goethitecrystal. Third, since the shape of goethite particle determines theshape of metal magnetic powder particle through subsequent processing ofthe goethite powder, it is preferred that the cobalt ions whichinfluence the growth rate of the goethite crystal are not present duringthe formation of the goethite particles.

SUMMARY OF THE INVENTION

Then, one object of the present invention is to provide a novel aciculariron/cobalt alloy base magnetic powder.

Another object of the present invention is to provide a method forproducing such novel acicular iron/ cobalt alloy base magnetic powder.

A further object of the present invention is to provide a magneticrecording medium comprising such novel acicular iron/cobalt alloy basemagnetic powder.

These and other object of the present invention are achieved by:

providing an acicular alloy base magnetic powder comprising iron andcobalt, which has an average particle long axis size of not larger than0.25 μm, an axial ratio of from 4 to 8, a cobalt content of from 8 to50% by weight based on the weight of iron and saturation magnetizationof at least 120 emu/g after being kept standing at the temperature of60° C. and relative humidity of 90% for a week; or

providing an acicular alloy base magnetic powder comprising iron andcobalt, which has an average particle long axis size of not larger than0.25 μm, a cobalt content of from 8 to 50% by weight based on the weightof iron and saturation magnetization of at least 120 emu/g after beingkept standing at the temperature of 60° C. and relative humidity of 90%for a week, and particle surfaces of which are covered with a ferritefilm comprising iron and cobalt; or

providing an acicular alloy base magnetic powder comprising iron andcobalt, which has an average particle long axis size of not larger than0.25 μm, a cobalt content of from 5 to 50% by weight based on the weightof iron and saturation magnetization of at least 120 emu/g after beingkept standing at the temperature of 60° C. and relative humidity of 90%for a week, and particle surfaces of which are covered with a filmcomprising at least one of silicon and aluminum.

The acicular magnetic powder may further comprise nickel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are electron microscopic photographs of Sample Nos. 1 and10, respectively.

DETAILED DESCRIPTION OF THE INVENTION

The present invention has been completed based on the followingfindings:

Goethite of a uniform particle form is formed and converted to magnetitehaving divalent iron ions. Then, a part of the divalent iron ions areexchanged with divalent cobalt ions, and cobalt is reacted to form asolid solution of iron and cobalt in which the particles form magnetite,whereby the acicular form of the particles is maintained and the contentof cobalt in the solid solution can be increased. Thereafter, theobtained particles are reduced to produce a novel alloy base magneticpowder. The novel alloy base magnetic powder has good acicular form andan average major (long axis) particle size of not larger than 0.25 μm.In addition, the novel alloy base magnetic powder has saturationmagnetization of at least 120 emu/g after being kept standing at thetemperature of 60° C. and relative humidity of 90% for a week. Thereason for this may be that cobalt can be present in the solid solutionstate although cobalt is contained in an amount at least 8% by weightbased on the weight of iron in the magnetic particles.

When the novel alloy base magnetic powder is contained in the magneticlayer of magnetic recording medium, it keeps good corrosion resistanceeven after being subjected to very severe recording and reproducingconditions. Such good property may be attributed to much higher hardnessof the alloy base magnetic particles than the metal iron magneticparticles because of a comparatively large amount of cobalt in the solidsolution of iron and cobalt in the particles.

In a preferred embodiment, nickel is present during synthesis of thecrystal of iron base particles. Nickel can further improve the corrosionresistance of the alloy base magnetic powder and contribute to thehomogeneous formation of solid solution of iron and cobalt.

That is, the iron salt and nickel salt are reacted in an aqueousalkaline solution to form a co-precipitated material of iron hydroxideand nickel hydroxide. Then, the co-precipitated material is oxidized togive iron base particles having a narrow distribution of particle size.In such case, when the amount of nickel is not smaller than 2% by weightbased on the weight of iron, the distribution of particle size of theiron base particles can be narrowed. The Ni-containing iron particlesare then reduced in a hydrogen atmosphere containing steam to formmagnetic iron oxide. Then, cobalt is reacted with the iron base magneticparticles according to the present invention and reduced. Thereby, theobtained alloy base magnetic powder has increased saturationmagnetization and corrosion resistance.

When the amount of nickel is smaller than 2% by weight based on theweight of iron, the distribution of particle size is not narrowed.Therefore, the increase of amount of cobalt to be subsequently addeddoes not necessarily achieve uniform concentration of cobalt in eachparticle or improvement of corrosion resistance.

In view of the durability of corrosion resistance, nickel is preferablypresent in a surface layer of each alloy base magnetic particle. Inaddition, nickel is preferably present in the form of an intermetalliccompound of iron and nickel.

To control the particle shape of the alloy base magnetic powder, metalelements other than iron, cobalt and nickel, for example, chromium andmanganese may be added. However, a large amount of such other metalelements decreases saturation magnetization. To prevent decrease ofsaturation magnetization, the total amount of iron and cobalt is notless than 90% by weight based on the total amount of all the metalelements.

To produce the alloy base magnetic powder having high saturationmagnetization and corrosion resistance, it is important to form thesolid solution of iron, cobalt and optionally nickel in which cobaltelements are homogeneously present in the other metal elements. If thecobalt elements are segregated on the surface of the alloy base magneticparticles, local cells are formed and corrosion is induced. Thereby,high saturation magnetization is not achieved.

To form the solid solution of iron, cobalt and nickel, for example, theNi-containing iron oxide magnetic powder is suspended in a solution ofcobalt chloride in a polyhydric alcohol, the suspension is heated toform the solid solution of cobalt in the iron oxide magnetic particles,and then the magnetic particles are heated and reduced with the hydrogengas (cf. Japanese Patent Kokai Publication No. 146900/1977).

In the above method, if the distribution of particle size of the ironoxide magnetic particles is broad, the particles have different specificsurface areas from each other, so that the rate of the cobalt exchangereaction differs among particles and the particles have differentcompositions from each other. Thereby, potential difference is formedamong the particles so that partial cells are formed and, in turn, thealloy base magnetic particles are corroded. Then, it is understood thatthe distribution of particle size plays an important role in theproduction of the magnetic powder having good corrosion resistance. Thatis, the distribution of particle size of goethite particles has to benarrow by the presence of nickel during the synthesis of the goethiteraw material to improve the corrosion resistance of the finally producedalloy base magnetic powder.

In the present invention, the control of the divalent iron ions isimportant to induce the exchange reaction between the divalent iron ionson the iron oxide magnetic particles and the cobalt ions so as to formthe solid solution of iron and cobalt.

The iron oxide which is used as a core crystal has the spinel structure,and the magnetite which has the largest content of Fe²⁺ (Fe²⁺ /Fe³⁺ =50%by weight) is represented by the formula: Fe³⁺ [Fe³⁺ Fe²⁺ ]O₄. Besides,

γ-Fe₂ O₃ containing no Fe²⁺ is represented by the formula: Fe³⁺ [Fe³⁺_(5/3) □_(1/3) ]O₄ which □ represents a hole. As the content of Fe²⁺increases, the number of holes decreases. In the reaction of cobalt withthe iron oxide in the polyhydric alcohol to form the solid solution, theexchange reaction between the Fe²⁺ ions on the surfaces of magnetic ionoxide particles and the Co²⁺ ions proceeds at the interface between theliquid and the solid, and the Co²⁺ ions trapped on the surfaces diffuseinto the inside of the particles through the holes.

Then, to add cobalt to the solid solution effectively in the abovemethod, the amount of Fe²⁺ or holes is controlled.

That is, when Fe²⁺ /Fe³⁺ is in the range between 5 and 45% by weight,the Fe²⁺ ions are replaced with the cobalt ions, and the cobalt ionsdiffuse into the particle inside. However, when Fe²⁺ /Fe³⁺ is less than5% by weight, the Fe²⁺ ions cannot be replaced with the cobalt ions, sothat the amount of cobalt in the solid solution decreases. When Fe²⁺/Fe³⁺ is more than 45% by weight, the amount of holes which cancontribute to the, diffusion of cobalt ions and the Co²⁺ ions diffusewith difficultly into the particle inside, so that cobalt segregates onthe particle surfaces, and the amount of cobalt in the solid solutiondecreases.

One of the general methods for controlling the amount of divalent ironions in the iron oxide magnetic powder comprises partially reducing theiron oxide magnetic powder in a hydrogen gas at high temperature.However, by such method, it is difficult to adjust the reductionconditions and then to achieve the desired ratio of Fe²⁺ /Fe³⁺. Inaddition, the iron oxide magnetic particles tend to be sinteredtogether, and the reaction rates of cobalt for the formation of solidsolution differ between the sintered parts and the unsintered parts.Therefore, the iron/cobalt alloy base magnetic powder which is producedby reducing the iron oxide magnetic powder containing cobalt in thesolid solution has poor erasing properties.

Japanese Patent Kokai Publication No. 90198/1978 discloses control ofthe content of divalent iron ions in the iron oxide magnetic powder tobe used as the raw material by dispersing the iron oxide magnetic powderhaving the controlled particle shape in the polyhydric alcoholcontaining the iron salt and heating the dispersion. This method isuseful to overcome the above problems.

That is, the amount of divalent iron ions in the magnetic iron oxidewhich has large influence on the content of cobalt is controlled in thesame polyhydric alcohol as that used for the reaction to form the solidsolution of iron and cobalt. When the resulting cobalt-containing ironoxide magnetic powder is used as the raw material, the particles areless sintered together during the production of the iron/cobalt alloyparticles.

When the magnetic iron oxide powder is dispersed in the polyhydricalcohol in which the iron salt is dissolved and heated, the divalentiron ions are introduced in the magnetic iron oxide particles to form anintermediate iron oxide. The amount of introduced divalent iron ionsincreases as the temperature rises or as the iron ion concentrationincreases.

Examples of the polyhydric alcohol are polyethylene glycol, ethyleneglycol, propylene glycol and glycerol.

The thermal reduction can be carried out under the known reductionconditions. The reduction temperature is usually from 150° to 500° C.

The conventional acicular magnetic powder has the axial ratio of atleast 10 in order to increase the coercive force through shapeanisotropy. If the axial ratio of the alloy base magnetic powdercomprising iron and cobalt and having the high saturation magnetizationis increased to 10 or higher, the coercive force is increased to about1700 0e or higher, but such coercive force is too high for the magneticrecording medium. To decrease the coercive force to about 1600 Oe orless, conventionally the particle size is increased. However, theincrease of particle size results in the mixture of particles havingvarious particle sizes from small to large and in turn broadening of theparticle size distribution. With the magnetic powder having suchbroadened particle size distribution, an anisotropic magnetic fielddistribution is broadened although the coercive force is adequate. Asthe result, the magnetic recording medium comprising such magneticpowder has deteriorated erasing properties. In addition, a magneticrecording medium generates large noise due to the large particle size ofthe magnetic powder.

To reduce the coercive force to a level suitable for the magneticrecording medium while keeping the particle size small and theanisotropic magnetic field distribution narrow, the axial ratio of themagnetic powder particle is adjusted in the range between 4 and 8,preferably between 4.5 and 7.0.

When the axial ratio is 4, the coercive force is about 0.78 times thatfor the axial ratio of larger than 10, and when the axial ratio is 8,the coercive force is about 0.96 times of that for the axial ratio oflarger than 10. When the axial ratio is less than 4, the control ofcoercive force is difficult. When the axial ratio is larger than 8, thecoercive force is not satisfactorily reduced.

Since the alloy base magnetic powder of the present invention hasreduced coercive force due to the small axial ratio, it is not necessaryto increase the particle size, and the average particle long axis sizeis preferably from 0.1 to 0.25 μm.

In any of the alloy base magnetic powder of the present invention, aratio of the half-width of the anisotropic magnetic field distributionto the coercive force is less than 3.2. This means that the anisotropicmagnetic field distribution is narrow. Then, the alloy base magneticpowder of the present invention has improved erasing properties, and themagnetic recording medium comprising such alloy base magnetic powdergenerates less noise.

The acicular alloy base magnetic powder of the present invention has thesaturation magnetization of 120 emu/g or larger and good corrosionresistance after being kept standing at the temperature of 60° C. andrelative humidity of 90% for a week. This is partially because of thepresence of cobalt in a suitable amount in the alloy base magneticpowder and partially because of the content of nickel in an amount of atleast 2% by weight based on the weight of iron.

The magnetic recording medium with good durability of corrosionresistance comprising the alloy base magnetic powder of the presentinvention can be prepared as follows.

The surfaces of the acicular particles of alloy base magnetic powder areexposed to hot oxygen-containing gas to form ferrite layers on theparticles. Alternatively, application of a layer containing at least oneof aluminum and silicon on the surfaces of the acicular alloy basemagnetic particle imparts corrosion resistance with durability to themagnetic power when the cobalt content is rather small. The aluminum- orsilicon-containing layer can be formed by dispersing the magneticparticles in a solution of an aluminum or silicon base compound andadding water to form the aluminum or silicon-containing layer throughhydrolysis. The effect of the aluminum- or silicon-containing layer canbe optimum when the amount of aluminum or silicon is in a specific rangesuch that the amount of the aluminum and/or silicon atoms is from 1 to 9atoms/nm². When the amount of such atoms is less than the lower limit,the corrosion resistance with durability cannot be effectively improved.When the amount of such atoms is larger than the upper limit, an amountof non-magnetic components undesirably increases so that the magneticproperties, particularly the saturation magnetization of the acicularalloy base magnetic powder tends to be deteriorated.

When the magnetic layer functions to protect alloy base magnetic powder,the corrosion resistance with durability of the magnetic powder isachieved. That is, as described later, such protection can be realizedby addition of inorganic powder having the Mohs' hardness of at least 5as a filler or addition of a lubricant which imparts lubricity to thesurface of the magnetic layer (e.g. aliphatic acids, aliphatic acidesters, mineral oils, etc.) or selection of a specific binder whichimparts abrasion resistance to the magnetic layer. When the surface ofthe magnetic layer has such roughness as 0.004 μm in terms of centerline average height, penetration of corrosive gas into the magneticlayer can be prevented and a coefficient of friction can be decreased,whereby formation of flaws on the surface of magnetic layer can bereduced. As a result, the corrosion resistance with durability of themagnetic recording medium can be improved.

When the magnetic recording medium is a magnetic tape, flaws which aregenerated by contact of the magnetic layer to a back coating when woundtend to deteriorate the corrosion resistance with durability. Therefore,the back coating preferably has a center line average height of 0.01 μmor less and contains primary particles or aggregates of carbon blackwhich has a particle size larger than the average particle long axissize of the alloy base magnetic powder.

With the above measures, the reliability of the recording medium ismaintained by maintaining the corrosion resistance even under severeabrasion and preventing an increase of drop out or noise. In the case ofa magnetic tape which comprises the magnetic layer and the back coating,the tape has a total thickness of 14 μm or less and is used in a woundstate.

In general, the magnetic recording medium is produced by applying amagnetic paint on a support film such as a polyester film and drying themagnetic paint to form the magnetic layer. The magnetic paint isprepared by dispersing the magnetic powder, the binder resin, theinorganic powder, the lubricant and the like in an organic solvent. Incase of a magnetic recording medium such as a video tape which has theback coating comprising the binder, the inorganic powder and thelubricant to improve traveling property, both the magnetic layer and theback coating contain the inorganic powder having the Mohs' hardness ofat least 5 to prevent abrasion caused by contact to a magnetic head orguide rollers.

The addition of the inorganic powder having the Mohs' hardness of atleast 5 to the back coating is proposed in Japanese Patent KokaiPublication Nos. 112/1987, 38525/1987, 38526/1987 and 38527/1987, andthe addition of such inorganic powder to the magnetic coating isproposed in Japanese Patent Publication Nos. 18572/1972, 15003/1973,39402/1974, 28642/1977, 49961/1977 and 15771/1980.

To further decrease the noise, the surface of the magnetic layer istreated by super calendering (cf. Japanese Patent Publication Nos.17404/1977 and 12688/1985).

When the magnetic recording medium is produced by using the acicularalloy base magnetic powder of the present invention, although theinitial noise is low, drop out or noise increases after a long travelingtime particularly after being stored under the corrosive conditions fora long time, particularly when the cobalt content in the magnetic powderis less than 8% by weight. That is, the drop out of signals having afrequency corresponding to the recording wavelength of 0.8 μm or less ornoise increases after a long traveling time.

This drawback can be overcome by the alloy base magnetic powder havingthe average particle long axis size of not larger than 0.25 μm and theaxial ratio of 4 to 8, containing cobalt in an amount of 8 to 50% byweight based on the weight of iron, the surface layer of ferritecomprising iron and cobalt and the layer comprising at least one ofaluminum and silicon on the particle surfaces. The magnetic recordingmedium comprising such alloy base magnetic powder keeps initial lownoise after storage.

Then, the above drawback may relate to not only the corrosion resistancebut also abrasion resistance of the alloy base magnetic powder, and theabove phenomenon is often seen in the magnetic tape having the totalthickness of 14 μm or less. One of the reasons for this is that thenumber of winding of such a thin magnetic tape per unit winding diameteris larger than that of a magnetic tape having a total thickness of about20 μm and the tape is wound under large force near a hub. Then, theabove drawback is particularly seen in the magnetic recording mediumcomprising the ferromagnetic alloy powder and having the centerrecording wavelength of 0.8 μm or less, while the shortest recordingwavelength at the white level is 1.3 μm in the VHS video tape comprisingthe support f11m, the magnetic layer and the back coating with the totalthickness of about 20 μm.

According to the present invention, it has been also found that thesusceptibility to flaws on the magnetic layer depends on the particlesize of the powder contained in the magnetic layer and that, althoughthe reinforcing effect on the magnetic layer is decreased when theparticle size is smaller than a certain size, the increased hardness ofpowder can maintain the strength of the magnetic layer. Based on thesefindings, as already described, the present invention proposes (1) thelimitation of the cobalt content in the alloy, (2) formation of thepassive state films on the particle surfaces and (3) application of theprotective layer comprising at least one of aluminum and silicon.

The corrosion resistance of the magnetic layer is influenced by the backcoating which seems to have no relation with the magnetic layer. Whenthe magnetic tape is wound, the inorganic powder, having the Mohs'hardness of at least 5 creates flaws in the surface of the magneticlayer, whereby the alloy base magnetic powder may be exposed to thecorrosive atmosphere. Therefore, preferably, the inorganic powder havingthe Mohs' hardness of at least 5 contained in the back coating has aparticle size smaller than the acicular alloy base magnetic powder.

To improve the traveling property of the magnetic recording medium, theback coating often contains carbon black.

It is known that increase of drop out is suppressed and the travelingproperty is improved by the use of fine grain carbon black having aparticle size of 10 to 30 mμ, coarse grain carbon black having aparticle size of 200 to 500 mμ and non-magnetic powder having a particlesize of not larger than 0.2 μm (cf. for example, Japanese Patent KokaiPublication No. 8328/1987).

However, none of the prior art suggests a relationship between theparticle size of primary particle or agglomerate of carbon black and theaverage particle long axis size of the ferromagnetic alloy powder in themagnetic layer. In addition, none of the prior art discloses therelationship between the noise against the signal frequencycorresponding to recording wavelength of not longer than 0.8 μm and theaverage particle when the acicular alloy base magnetic powder has asmall average particle long axis size.

During the development of the present invention, the relationshipbetween the particle size of primary particles or agglomerate of carbonblack and the average particle long axis size of the ferromagnetic alloypowder in the magnetic layer was studied to find that when the particlesize of primary particles or agglomerate of carbon black is larger thanthe average particle long axis size of the ferromagnetic alloy powder,the high output and low noise are achieved.

The above particle size relationship is important to satisfy therequirement for high density recording, particularly in the magneticrecording medium comprising the magnetic layer with a thickness of about3.0 μm or less.

To provide the magnetic recording medium which comprises the magneticlayer and the back coating on the respective surfaces of the supportfilm, achieving high output and suppressing the noise as much aspossible in the high density recording with the center recordingwavelength of 0.8 μm or less, it is preferred that the magnetic layerhas a squareness ratio of at least 0.85 measured at the applied magneticfield of 10 KOe, the magnetic layer has the coercive force of at least1500 Oe, the surface of magnetic layer has the smoothness of 0.004 μm interms of the center line average height, the surface of the back coatinghas the smoothness of 0.01 μm or less in terms of the center lineaverage height, and the particle size of inorganic powder having theMohs' hardness of at least 5 is smaller than the average particle longaxis size of the alloy base magnetic powder.

Since carbon black is rather soft in general, it could not be expectedthat carbon black in the back coating would cause flaws in the surfaceof the magnetic layer. To achieve the high density recording with thecenter recording wavelength of 0.8 μm or less, the average particle longaxis size of magnetic particles is made 0.2 μm or less and the magneticlayer comprising the metal magnetic powder has insufficient reinforcingstrength. In addition, the thin recording tape required for the highdensity recording is wound many times which generates strong windingforce. Therefore, the small particles of carbon black tend to causeflaws in the surface of the magnetic layer. In such a case, when theback coating contains carbon black having the particle size of primaryparticles or agglomerate of carbon black which is larger than theaverage particle long axis size of the ferromagnetic alloy powder, thesmall particles are present in spaces among the large ones so that thesmall particles do not appear on the surface of the back coating.Preferably, the inorganic powder having the Mohs' hardness of at least 5has the particle size smaller than the average particle long axis sizeof acicular alloy base magnetic powder. When the magnetic tape havingthe above construction is wound around the hub made of, for example,polyoxymethylene resin with the back coating facing inside, the woundtape is preferably stored in a cassette casing made of ABS resincontaining a pigment having the Mohs' hardness of 2 to 4.

In the present invention, the non-magnetic support film may be any oneof conventionally used resin films. Examples of the resins arepolyesters (e.g. polyethylene terephthalate, polyethylene2,6-naphthalate, etc.), polyolefins (e.g. polyethylene, polypropylene,etc.), cellulose derivatives (e.g. cellulose triacetate, cellulosediacetate, etc.), vinyl resins (e.g. polyvinyl chloride, polyvinylidenechloride, etc.), polycarbonates, polyimides, polyamides, and the like.Among them, preferred are biaxial orientation type polyesters such aspolyethylene terephthalate or polyethylene 2,6-naphthalate havingmodulus in the.. longitudinal direction of at least 700 kg/mm² and inthe widthwise direction of at least 400 kg/mm² and surface roughness of0.01 μm in terms of center line average height.

On one surface of the support film, the magnetic paint comprising theorganic solvent, the magnetic powder, the inorganic powder having theMohs' hardness of at least 5 and the binder resin as well as otheroptional additives are applied in a desired thickness and dried whilebeing subjected to the magnetic field orientating treatment to form themagnetic layer.

In general, the average particle long axis size of the alloy basemagnetic powder is 0.8 μm or less in view of the relationship with thecenter recording wavelength, and preferably 0.25 μm or less in view ofresolving power.

In case of the alloy base magnetic powder containing nickel and cobalt,when the alloy composition is adjusted so that the nickel content is atleast 2% by weight and the cobalt content is at least 110% by weightbased on the weight of nickel, the saturation magnetization of themagnetic powder can be kept at not less than 120 emu/g after being keptstanding at the temperature of 60° C. and relative humidity of 90% forone week. In view of erasing properties, the ratio of the half-width ofthe anisotropic magnetic field distribution to the coercive force ispreferably less than 3.2.

Examples of the inorganic powder having the Mohs' hardness of at least 5are metal oxides, metal carbides, metal nitrides, etc. Among them, α-Fe₂O₃ (6), Al₂ O₃ (9), Cr₂ O₃ (9), SiO₂ (6), TiO₂ (6), ZrO₂ (6), SiC (9),TiC (9), hBN (9) and Si₃ N₄ (9) are more preferred, wherein the valuesin the parentheses indicate the Mohs' hardness of the respectiveinorganic materials. The organic powder having various particle sizescan be commercially available.

Examples of carbon black to be contained in the back coating are channelblack, furnace black, acetylene black, thermal black, etc. Among them,acetylene black is preferred. The graphited carbon black, namely carbonblack coated with graphite may be used (cf. Japanese Patent KokaiPublication No. 22424/1986).

Commercially available carbon black are Black Pearl 700 (particle sizeof 18 mμ), Mogal L (particle size of 20 mμ), ELFTEX pellets-115(particle size of 27 mμ), Regal 3001 (particle size of 27 mμ), VulcanXC-72 (particle size of 30 mμ), Sterling NS and R (particle size of 75mμ) (manufactured by Cabott); Laben 8000 (particle size of 13 mμ), Laben5250 (particle size of 20 mμ), Laben 450 (particle size of 62 mμ), Laben410 (particle size of 70 mμ), Laben MT-P beads (particle size of 280mμ), Laben Sebacalb MT-CI (particle size of 300 mμ) (manufactured byColumbian Carbon); HS-500 (particle size of 75 mμ), #60H (particle sizeof 35 mμ) (manufactured by Asahi Carbon); Seest 5H (particle size of 20mμ) (manufactured by Tokai Carbon); Ketchen Black EC (manufactured byAkzo); and #4040 (particle size of 20 mμ), #4330BS (particle size of 23mμ), #4350BS (particle size of 45 mμ), #4010 (particle size of 80 mμ)(manufactured by Mitsubishi Chemical).

As above, the carbon blacks with various particle sizes are commerciallyavailable and selected according to the relationship with the averageparticle long axis size of the alloy base magnetic powder according tothe present invention. When the carbon black has the comparatively smallparticle size, it is preferred to form an agglomerate consisting of theprimary particles by using the structure forming ability of the carbonblack. The agglomerate of carbon black particles acts as a singleparticle.

The inorganic powder and the carbon black are added to the magneticpaint or the back coating paint in such amounts that the above particlesize relationships are fulfilled.

Examples of the binder resin to be contained in the magnetic layer orthe back coating are vinyl resins (e.g. vinyl chloride-vinyl acetatecopolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinylchloride-acrylate copolymer, vinyl chloride-vinylidene chloridecopolymer, vinyl chloride-acrylonitrile copolymer, vinyl chloride-vinylacetate-maleic acid copolymer, etc.), thermoplastic polyurethane resins,thermosetting polyurethane resins, polyester resins, phenoxy resins,polyvinyl butyral resins, cellulose derivatives, epoxy resins, andmixtures thereof. In these resins, hydrophilic groups such as thosederived from carboxylic acids, sulfonic acids, sulfonate salts,phosphoric acid, phosphoric salts, amines, ammonium salts, etc. can beintroduced so as to improve dispersibility of the powder particles.Further, an acrylic double bond is introduced in the molecule to curethe resin through radiation of electron beams.

Examples of the solvent to be used in the preparation of the magneticpaint and the back coating paint are alcohols (e.g. ethanol, propanol,butanol, etc.), esters (e.g. methyl acetate, ethyl acetate, butylacetate, etc.), ketones (e.g. methyl ethyl ketone, methyl isobutylketone, cyclohexanone, etc.), ethers (e.g. tetrahydrofuran, dioxane,etc.), aromatic hydrocarbons (e.g. benzene, toluene, xylene, etc.),aliphatic hydrocarbons (e.g. heptane, hexane, cyclohexane, etc.) andchlorinated hydrocarbons (e.g. methylene chloride, ethylene chloride,chloroform, etc.) as well as mixtures thereof. Among them, a mixedsolvent of cyclohexanone and toluene is preferred.

The paints may contain a lubricant such as saturated or unsaturatedhigher fatty acids, higher fatty acid amides, fatty acid esters, higheralcohols, silicone oils, mineral oils, food oils, fluorine-containingoils, etc.

The above components in desired amounts are dispersed in a ball mill ora sand mill to prepare the magnetic paint or the back coating paintwhich is then applied on the surface of non-magnetic support film.

The mixture of the components should be carefully dispersed so as not toapply excessive force to the inorganic powder or carbon black.Otherwise, the particle size of carbon black would be changed.

For the application of paints, the magnetic paint is firstly coated onone surface of the support film, subjected to magnetic field orientationand then dried followed by surface smoothing treatment. After windingthe support film having only the coated magnetic layer, the back coatingpaint is coated on the other surface of the support film while unwindingthe support film. These steps are described in detail in Japanese PatentPublication No. 23647/1983. Alternatively, the magnetic layer and theback coating can be formed simultaneously.

Each of the magnetic layer and the back coating is formed by oneapplication of the paint or plural applications of the paint.

The support film having the magnetic layer and the back coating is cutwith a slitting apparatus to form a tape with a desired width. Then, thetape is wound around a hub with the back coating inside and set in acassette casing.

When the total thickness of the magnetic layer, the support film and theback coating is not larger than 14 μm, the modulus in the longitudinaldirection is larger than that in the widthwise direction but not largerthan twice the modulus in the widthwise direction. In addition, themodulus in the longitudinal direction is at least 1000 kg/mm².

The magnetic layer has hardness such that when the tape is traveled for23 meters while pressing a steel ball of 6 mm in diameter under load of5 grams, the volume decrease of the steel ball is not larger than20×10⁻⁵ mm³. When the magnetic layer has such hardness, it is not flawedby reciprocally sliding a sapphire blade having a vertical angle of 45°on the magnetic layer under load of 50 g/3.81 mm width for 1000 times.

The above relationship between the modules in the longitudinal andwidthwise directions is achieved by selecting the suitable supportmaterial and/or mechanical control of the magnetic particles throughmagnetic field orientation. The modulus of 1000 kg/mm² or larger in thelongitudinal direction is achieved by selecting the curing conditions ofthe binder resins in the magnetic layer and the back coating.

To regulate the volume decrease of the steel ball with the magneticlayer to 20×10⁻⁵ mm³ or less, the amount of the inorganic powder havingthe Mohs' hardness of at least 5, the conditions for smoothing thesurface and the drying conditions of the paints are adequately adjusted.

Since a strong winding force is applied to the hub during winding of thetape, a depth of a sink mark on the peripheral surface of the hub onwhich the tape is wound should be less than 5 μm in order to preventcurling of the tape.

Since the back coating side of the tape contacts the tape guides of thecassette casing, at least the tape guide parts are preferably made ofthe ABS resin to decrease the noise. To prevent the flaws of the backcoating, the pigment having the Mohs' hardness of 2 to 4 such as calciumcarbonate and barium sulfate is added to the resin from which the tapeguides are made. To prevent the electrostatic noise, the resin formaking the cassette casing preferably contains an antistatic agent suchas carbon black and a quaternary ammonium salt (e.g. polyoxyethylenealkylamine) and a fluidizing agent for the molten resin such asethylene-bis-stearoamide. The cassette casing can be produced by meltingthe resin containing the above additives and injection molding themolten resin in a mold.

PREFERRED EMBODIMENTS OF THE INVENTION

The present invention will be explained further in detail by thefollowing examples in which "parts" are be weight unless otherwiseindicated.

I. Preparation of Alloy Base Magnetic Powder

To a 5 mol/l aqueous solution of sodium hydroxide (1.5 l), an aqueoussolution of ferrous sulfate (0.72 mol/l) and nickel sulfate (A mol/l,see below Table 1) (1.5 l) was added at room temperature while stirringto co-precipitate ferrous hydroxide and nickel hydroxide. In thesuspension of co-precipitated material, air was bubbled at a flow rateof 1.6l/min. at 40° C. for 8 hours while stirring. The resulting mixturewas filtered, washed with water and dried to obtain acicular goethiteparticles having the average particle long axis size of 0.22 μm and theaxial ratio of 7. The particle shape varied with the concentration ofthe alkaline solution and the metal salts, and the acicular ratio (G)could be changed by slightly changing these concentrations (see Table1). The goethite particles (100 g) were dispersed in water (3 liters).To the suspension, a 1 mol/l aqueous solution of sodium hydroxide (2liters) and a 1 mol/l aqueous solution of sodium orthosilicate (26 ml)were added followed by blowing carbon dioxide gas until the pH of thesuspension reached 8. The particles were washed with water and dried tocoat the particle surfaces with a silicate compound.

Then, the iron oxide particles coated with the silicate compound weredispersed in water (3 liters). To the suspension, a 1 mol/l aqueoussolution of sodium hydroxide (2 liters) and a 0.5 mol/l solution ofsodium aluminate (135 ml) were added followed by blowing carbon dioxidegas until the pH reached 8. The particles were washed with water anddried to coat the particle surfaces with alumina. Then, the goethiteparticles coated with the silicate compound and alumina were sintered at750° C. for 4 hours and then reduced in a hydrogen stream containingwater vapor at 300° C. for 8 hours to obtain magnetic iron oxide powder.The Fe²⁺ /Fe³⁺ ratio in the iron oxide was adjusted by one of thefollowing ways:

(B-i) The magnetic iron oxide powder (20 g) was heated and partiallyoxidized in an oxygen-containing atmosphere to adjust the Fe²⁺ /Fe³⁺ratio at D% (see Table 1).

(B-ii) In a solution of ferrous chloride tetrahydrate (C g, see Table 1)in ethylene glycol (300 g), the iron oxide powder (40 g) was dispersedand heated at 180° C. for 4 hours while stirring to adjust said ratio atD%, followed by washing with water and drying.

In a solution of cobalt chloride hexahydrate (see Table 1) inpolyethylene glycol (300 ml), the magnetic iron oxide having thecontrolled Fe²⁺ /Fe³⁺ ratio (20 g) was dispersed and heated at 200° C.for 6 hours while stirring to form the magnetic iron oxide powder inwhich cobalt was homogeneously present in the form of a solid solution.After washing with water, the magnetic powder was reduced in a hydrogenatmosphere at 450° C. for 2 hours and then gradually oxidized at 60° C.for 2 hours while the flow of an inert gas nitrogen, helium or argon)containing 1000 ppm of oxygen to obtain the acicular alloy base magneticpowder having the average particle long axis size of 0.2 μm and aferrite layer comprising iron and cobalt on the particle surfaces.

The surfaces of acicular alloy base magnetic particles were treated asfollows (F):

To the suspension of acicular alloy base magnetic powder (100 g) inethanol (2 liters), Si(OC₂ H₅)₄ (7.5 g) was added. After heating themixture to 60° C., water (7.8 g) was dropwise added to hydrolyze Si(OC₂H₅)₄ to form a layer of the hydroxide of silicon in an amount of 7 Siatoms/nm² on the particles surfaces of the acicular alloy base magneticpowder.

The above process conditions A through G employed in the production ofSample Nos. 1 through 14 are summarized in Table 1.

                                      TABLE 1                                     __________________________________________________________________________        nickel                                                                             Fe.sup.2+ /Fe.sup.3+                                                                Ferrous    Cobalt                                              Sample                                                                            sulfate                                                                            adjustment                                                                          chloride                                                                           Fe.sup.2+ /Fe.sup.3+                                                                chloride                                                                           Surface                                                                             Axial                                    No. A mol/l                                                                            method B                                                                            C g  D wt %                                                                              E g  treatment F                                                                         ratio G                                  __________________________________________________________________________    1   0.03 (i)   --   20    25   Yes   7                                        2   0.03 (i)   --   20    30   Yes   7                                        3   0.03 (i)   --   20    21   Yes   7                                        4   0.03 (i)   --   20    10   No    7                                        5   0.02 (i)   --   20    25   Yes   7                                        6   0    (i)   --   20    25   Yes   7                                        7   0.03 (i)   --   15    25   Yes   7                                        8   0.03 (i)   --   25    25   Yes   7                                        9   0.03 (i)   --   3     25   Yes   7                                        10  0.03 (i)   --   46    25   Yes   7                                        11  0.03 (ii)  4.0  20    25   Yes   7                                        12  0.03 (ii)  0.2  2     25   Yes   7                                        13  0.03 (i)   --   20    25   Yes   10                                       14  0.03 (i)   --   20    10   Yes   7                                        __________________________________________________________________________

II. Formation of Magnetic Layer

The acicular alloy base magnetic powder prepared in the above (100parts), hydroxyl group-containing polyvinyl chloride base resin having apolymerization degree of 340 (10 parts), thermoplastic polyurethaneresin (7 parts), alumina having the particle size of 0.2 μm (8 parts),myristic acid (2 parts), red oxide (α-Fe²⁰³) having the particle size of0.8 μm (2 parts) and carbon black (one part of #4010 manufactured byMitsubishi Chemical having the particle size of 20 mμ and 2 parts ofSeest 5H manufactured by Tokai Carbon having the particle size of 20 mμ)were kneaded with the mixed solvent of cyclohexanone (70 parts) andtoluene (70 parts) in a ball mill for 96 hours. To the mixture, atrifunctional polyisocyanate compound (5 parts) was added and mixed toprepare a magnetic paint. The magnetic paint was coated on one surfaceof a biaxially orientated polyethylene terephthalate film having thethickness of 10 μm in the dry thickness of 2.5 μm and dried followed bycalendering to form the magnetic layer.

III. Formation of Back Coating

Carbon black (60 parts of Seest 5H manufactured by Tokai Carbon havingthe particle size of 20 μm and 7.5 parts of Laben MT-P beadsmanufactured by Columbian Carbon having L the particle size of 280 mμ),calcium carbonate having the particle size of 0.05 μm (30 parts), redoxide having the particle size of 0.1 μm (2.5 parts), thermoplasticpolyurethane resin (45 parts), nitrocellulose (40 parts) and atrifunctional isocyanate compound (15 parts) as a cross linking agent)were kneaded with the mixed solvent of cyclohexanone (330 parts) andtoluene (330 parts) in a ball mill for 96 hours to prepare a backcoating paint. The back coating paint was coated on the other surface ofthe polyethylene terephthalate film having the magnetic layer on onesurface in the dry thickness of 1.0 μm and dried followed by curing at60° C. for 20 minutes.

The total thickness of the coated film was 13.5 μm.

The coated film was cut to obtain a magnetic tape having the desiredwidth and wound around a hub made by injection molding ofpolyoxymethylene resin with the back coating layer side inside. Theperipheral surface of the hub on which the tape is wound had sink marksof less than 0.1 μm in depth.

IV. Production of Cassette Casing

An ABS resin (NA-1060 manufactured by Denkikagaku Industries) (100parts), a colorant consisting of carbon black treated with ferrocyanineblue (23 parts), calcium carbonate having the particle size of 0.5 μm(35 parts), polyoxyethylene alkylamine (12 parts) andethylene-bis-stearoamide (3 parts) were kneaded in a Henschel mixer at110° C. for one minute and extruded with a twin-screw extruder at 220°C. to produce pellets. Then, the pellets were melt together with thesame ABS resin (1500 parts) in a furnace at 240° C. and injection moldedat a molding temperature of 30° C. to produce a cassette casing.

In the cassette casing, the tape wound around the hub was set toassemble a cassette tape.

All the cassette tapes had the squareness ratio of at least 0.85measured in the applied magnetic field of 10 KOe. The center lineaverage height of each of the magnetic layer and the back coating wasmeasured with a tracer type surface roughness tester with using a tracerhaving R of 2 μm at a cut off of 0.08 mm. The magnetic layer and theback coating had the center line average heights of 0.004 μm and 0.01μm, respectively.

The ratios of cobalt, nickel, aluminum and silicon to iron, namelyCo/Fe, Ni/Fe, Al/Fe and Si/Fe (all in wt %) in the magnetic powder ofeach of Sample Nos. 1 through 14 were measured by the fluorescent X-rayspectroscopy, and the axial ratios were measured by a transmission typeelectron microscope. With a sample vibration type magnetometer, coerciveforce (Oe), saturation magnetization (emu/g), saturation magnetizationafter being kept standing at the temperature of 60° C. and relativehumidity of 90% for one week (emu/g), and the anisotropic magnetic fielddistribution were measured. The anisotropic magnetic field distributionis expressed in terms of the ratio of the half-width of the anisotropicmagnetic field distribution to the coercive force. The results are shownin Table 2.

                                      TABLE 2                                     __________________________________________________________________________                                         Saturation                                                                            Anisotropic                                              Coercive                                                                           Saturation                                                                            magnetization                                                                         magnetic                         Sample                                                                            Co/Fe                                                                             Ni/Fe                                                                             Al/Fe                                                                             Si/Fe                                                                             Axial                                                                             force                                                                              magneticzation                                                                        after one week                                                                        field                            No. (wt %)                                                                            (wt %)                                                                            (wt %)                                                                            (wt %)                                                                            (wt %)                                                                            (Oe) (emu/g) (emu/g) distribution                     __________________________________________________________________________    1   21.0                                                                              4.2 3.5 1.6 7.0 1550 158.0   137.5   2.3                              2   25.0                                                                              4.4 3.6 1.6 7.0 1590 161.1   145.1   2.5                              3   17.3                                                                              4.0 3.6 1.5 7.0 1501 156.2   138.0   2.3                              4   8.1 4.1 3.4 0.6 7.0 1500 151.0   110.0   2.4                              5   20.3                                                                              3.0 3.6 1.5 7.0 1500 157.3   137.9   2.6                              6   20.9                                                                              0.0 3.5 1.5 7.0 1420 160.9   105.0   3.3                              7   18.2                                                                              4.1 3.5 1.6 7.0 1513 156.5   138.2   2.3                              8   26.3                                                                              4.0 3.5 1.5 7.0 1591 161.0   143.3   2.3                              9   6.0 4.2 3.6 1.5 7.0 1314 140.0   125.5   2.5                              10  5.2 4.1 3.6 1.6 7.0 1298 139.8   123.3   2.5                              11  20.0                                                                              4.2 3.5 1.5 7.0 1541 156.1   136.8   2.3                              12  5.9 4.1 3.5 1.5 7.0 1300 138.9   124.1   2.6                              13  21.0                                                                              4.1 3.5 1.6 9.0 1753 157.8   136.2   2.8                              14  8.1 4.1 3.4 1.5 7.0 1527 150.0   131.8   2.5                              __________________________________________________________________________

With the magnetic recording media produced with using each of SampleNos. 1 through 8 and 14, the decrease (%) of saturation magnetic fluxdensity after being kept standing at the temperature of 60° C. andrelative humidity of 90% for one week was measured. Also, the magnetictape slit to the width of 3.81 mm was abraded by reciprocally sliding asapphire blade having a vertical angle of 45° on the magnetic layerunder the load of 50 g/3.81 mm width for 1000 times and immediately keptstanding at the temperature of 60° C. and relative humidity of 90% forone week. Thereafter, the decrease of saturation magnetic flux densitywas measured with the VSM.

Signals of 7 MHz was recorded on the magnetic tape and the noise levelat 6 MHz during reproducing was measured just after wound around thehub, or after being slitting the tape to the width of 3.81 mm, abradingthe slit tape by reciprocally sliding a sapphire blade having a verticalangle of 45° on the magnetic layer under the load of 50 g/3.81 mm widthfor 1000 times and immediately being kept standing at the temperature of60° C. and relative humidity of 90% for one week. The noise level afterabrading and being kept standing at the temperature of 60° C. andrelative humidity of 90% for one week was compared with the noise leveljust after being wound around the hub (0 dB).

The results are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                           Degrease of sat-                                                 Decrease     uration magnetization                                            of saturation mag-                                                                         density after abrasion                                     Sample                                                                              netic flux density                                                                         and kept standing                                                                            Noise level                                 No.   after abrasion (%)                                                                         for one week (%)                                                                             change (dB)                                 ______________________________________                                        1     -2.0         -2.0           0.1                                         2     -2.0         -2.0           0.1                                         3     -2.5         -2.6           0.1                                         4     -12.0        -14.1          2.0                                         5     -2.2         -2.3           0.2                                         6     -11.3        -14.4          2.3                                         7     -2.0         -2.3           0.2                                         8     -2.0         -2.1           0.1                                         14    -3.8         -4.0           0.2                                         ______________________________________                                    

With the magnetic powder of Sample Nos. 1 and 10, the particle surfaceswere observed with the transmission type electron microscope. Thethickness of the ferrite layer was 28 Å for Sample No. 1 and 35 Å forSample No. 10. The electron microscopic photographs of Sample Nos. 1 and10 are shown in FIGS. 1 and 2, respectively.

From the above results, it is understood that the acicular alloy basemagnetic powder of the present invention has large saturationmagnetization and good corrosion resistance, and also the magneticrecording medium comprising such magnetic powder was improved corrosionresistance and corrosion resistance with durability. The noise level isnot increased after the test for corrosion resistance with durabililty.

Therefore, the magnetic recording medium comprising the acicular alloybase magnetic powder of the present invention is suitable for recordingin the short wavelength range.

What is claimed is:
 1. A magnetic recording medium comprising a supportfilm and a magnetic layer containing acicular alloy bas magnetic powderparticles comprising nickel, iron and cobalt in a binder, said magneticpowder having an average long axis particle size of not greater than0.20 μm, such that a ratio of a half-width of an anisotropic magneticfield distribution to a coercive force is not larger than 3.2, an axialratio of from 4 to 8, a cobalt content of from 5 to 50% by weight basedon the wight of iron, a nickel content in an amount of at least 2% byweight based on the wight of iron, and a saturation magnetization of atleast 120 emu/g after being kept standing at a temperature of 60° C. andrelative humidity of 90% for a week, particle surfaces of which arecovered with a ferrite film comprising iron and cobalt and furtherincluding a film comprising at least one of aluminum and silicon.
 2. Amagnetic recording medium according to claim 1, wherein said saturationmagnetization decreases only by 10% or less after said magnetic layer ofsaid recording medium has been abraded and left standing at atemperature of 60° C. and relative humidity of 90% for a week.
 3. A Themagnetic recording medium of claim 1, wherein nickel is locally presentin surface parts of said alloy particles.
 4. A The magnetic recordingmedium of claim 1, wherein nickel is present in said alloy particles inthe form of a solid solution of iron and nickel.
 5. The magneticrecording medium of claim 1, wherein the ferrite film has a thickness of5 to 30A.
 6. The magnetic recording medium of claim 1, wherein thecontent of cobalt is form 10 to 30% by weight based on the weight ofiron.
 7. The magnetic recording medium of claim 1, wherein a coatedamount of at least one of silicon and aluminum is from 1 to 9 atoms/nm².8. A magnetic recording medium for recording signals having a centerrecording wavelength of 0.8 μm or less, which medium comprises a supportfilm, one surface of said support film having a magnetic layer coatedthereon containing acicular alloy base magnetic powder particlescomprising nickel, iron and cobalt in a binder, said magnetic powderhaving an average particle long axis size of not greater than 0.20 μm,such that a ratio of a half-width of an anisotropic magnetic fielddistribution to a coercive force is not larger than 3.2, an axial ratioof from 4 to 8, a cobalt content of from 5 to 50% by weight based on theweight on iron, a nickel content in an amount of at least 2% by weightbased on the weight of iron, and a saturation magnetization of at least120 emu/g after being kept standing at a temperature of 60° C. andrelative humidity of 90% for a week, particle surfaces of which arecovered with a ferrite film comprising iron and cobalt and furtherincluding a film comprising at least one of aluminum and silicon, and aninorganic powder having a Mohs' hardness of at least 5, and an oppositesurface of said support film having a back coating containing primaryparticles of aggregates of carbon black having a particle size largerthan the average particle long axis size of said acicular alloy basemagnetic powder, wherein said magnetic layer has a surface smoothness of0.004 μm or less in terms of center line average height and a squarenessratio of at least 0.85 measured in an applied magnetic field of 10 KOe,and said back coating has a surface smoothness of 0.01 μm or less interms of center line average height.
 9. The magnetic recording mediumaccording to claim 8, wherein said magnetic layer, support film and backcoating is not greater than 14 μm is total thickness.
 10. The magneticrecording medium according to claim 8, wherein the ferrite film has athickness of 5 to 20A.
 11. The magnetic recording medium according toclaim 8, wherein the content of cobalt is from 10 to 30% by weight basedon the weight of iron.
 12. The magnetic recording medium according toclaim 8, wherein a coated amount of at least one of silicon and aluminumis from 1 to 9 atoms/nm².